Engineering topics

Wahr, John M.

Publications and source records attributed to Wahr, John M..

7 records

NASA NTRS1993

A study of the earth's free core nutation using international deployment of accelerometers gravity data

In this study we consider the influence of the earth's free core nutation (FCN) on diurnal tidal admittance estimates for 11 stations of the globally distributed International Deployment of Accelerometers network. The FCN causes a resonant enhancement of the diurnal admittances which can be used to estimate some properties of the FCN. Estimations of the parameters describing the FCN (period, Q, and resonance strength) are made using data from individual stations and many stations simultaneously. These yield a result for the period of 423-452 sidereal days, which is shorter than theory predicts but is in agreement with many previous studies and suggests that the dynamical ellipticity of the core may be greater than its hydrostatic value.

Cummins, Phil R.↗

NASA NTRS1991

The effects of the solid inner core and nonhydrostatic structure on the earth's forced nutations and earth tides

This paper computes the effects of the solid inner core (IC) on the forced nutations and earth tides, and on certain of the earth's rotational normal modes. The theoretical results are extended to include the effects of a solid IC and of nonhydrostatic structure. The presence of the IC is responsible for a new, almost diurnal, prograde normal mode which involves a relative rotation between the IC and fluid outer core about an equatorial axis. It is shown that the small size of the IC's effects on both nutations and tides is a consequence of the fact that the IC's moments of inertia are less than 1/1000 of the entire earth's.

De Vries, Dan↗

NASA NTRS1991

Report of the panel on earth rotation and reference frames, section 7

Objectives and requirements for Earth rotation and reference frame studies in the 1990s are discussed. The objectives are to observe and understand interactions of air and water with the rotational dynamics of the Earth, the effects of the Earth's crust and mantle on the dynamics and excitation of Earth rotation variations over time scales of hours to centuries, and the effects of the Earth's core on the rotational dynamics and the excitation of Earth rotation variations over time scales of a year or longer. Another objective is to establish, refine and maintain terrestrial and celestrial reference frames. Requirements include improvements in observations and analysis, improvements in celestial and terrestrial reference frames and reference frame connections, and improved observations of crustal motion and mass redistribution on the Earth.

Dickey, Jean O.↗

NASA NTRS1988

The theory of the earth's orientation, with some new results for nutation

The rotation of the earth is variable at a number of time scales, from a few days to thousands of years and longer. Variations occur in the rotation rate, in the geographical position of the rotation axis (referred to as polar motion), and in the position of the axis relative to inertial space (referred to as nutation and precession). The interpretations of the various observations have implications for the dynamical behavior and structure of the earth's deep interior, and for various aspects of meteorology and oceanography. These are reviewed below. A model of the diurnal resonance in nutation for a nonhydrostatically pre-stressed earth is also presented.

Wahr, John M.↗

NASA NTRS1987

The earth's C21 and S21 gravity coefficients and the rotation of the core

Observational results for the earth's C21 and S21 gravity coefficients can be used to constrain the mean equatorial rotation of the core with respect to the mantle. Current satellite gravity solutions suggest the equatorial rotation rate is no larger than 1 x 10 to the -7th times the earth's diurnal spin rate, a limit more than one order of magnitude smaller than the polar rotation rate inferred from the westward drift of the earth's magnetic field. The next generation gravity solutions should improve this constraint by more than one order of magnitude. Implications for the fluid pressure at the core-mantle boundary and for the shape of that boundary are discussed.

Wahr, John M.↗

NASA NTRS1986

Topographic effect on tilt, strain, and displacement measurements

An approximate analytical method is used to give simple expressions for the effect of arbitrarily shaped topography on vertical, horizontal and surface tilt measurements. Corrections to tilt fields are found to be directly proportional to the local slope of the topography. Comparison is made between expressions for tilt and those for strain and displacement. Although the topographic effects on secular tilt, strain, and vertical displacement are not yet measurable, the topographic effect on precision tidal tilt and strain measurements can be significant.

Meertens, Charles M.↗

NASA NTRS1986

Polar motion-induced gravity

Variations in the geocentric position of the earth's rotation axis (polar motion) cause deformation within the earth. The effects of this deformation on surface gravity and on radial and horizontal positions of points on the earth's surface are estimated. The effects of the oceans and of the earth's anelasticity on this deformation are found to be negligible. Peak-to-peak variations in surface gravity of 10 microgals or more, and in radial motion of 1-2 cm are possible over six months or so. These numbers are small enough that they can probably not be used to learn about the earth; however, they are large enough to noticeably affect present high-quality geodetic observations.

Wahr, John M.↗